Walshe replaces Peter Sherman, who left BBDO last month to join JWT New York as its chief executive.
He will be responsible for BBDO’s operation in 27 European markets and 46 agencies. These exclude the UK, France, Germany and Russia, which report directly to Andrew Robertson, the president and chief executive of BBDO Worldwide.
Walshe, who will report to Robertson, joined BBDO in 1997 after graduating from Trinity College in Dublin. He will continue to handle the Mars business alongside his new responsibilities in Europe.
Robertson said: "St John is a true internationalist. His leadership on our Mars business has been exceptional and a key driver of its growth. These new responsibilities are a reflection of St John’s important contributions to our business."